The present invention relates to liquid crystal materials for use in liquid crystal display elements, and to substances useful for these components. More particularly, the present invention relates to new chiral materials useful for components of ferroelectric liquid crystal mixtures, and to liquid crystal mixtures containing these materials.
Display element of a twisted nematic (TN) type is widely used as a liquid crystal display element. The liquid crystal display is characterized in less eyestrain and minimized consumption of electric power because it is a non-emissive type. However, the display element of the TN type has an unsolved disadvantage in which the response is slower than that of a display element of light emitting type (e.g. an electroluminescence display, a plasma display and the like). The improvement of the response time of the liquid crystal display is tried in many ways. As a liquid crystal display using a different principle instead of the TN type display, a display method using a ferroelectric liquid crystal is reported by N. A. Clark et al. (ref. Appl. Phys. Lett. 36, 899 (1980)).
This display method utilizes a chiral smectic C phase (abbreviated as S.sub.C * phase hereinafter) or a chiral smectic H phase (abbreviated as S.sub.H * phase hereinafter) of ferroelectric liquid crystals, and it has three excellent characteristics in comparison with the TN display method. The first characteristic is quick response and the response time is less than 1/100 of that of the TN display element. The second characteristic is that the method has memory effect and it facilitates multiplexed drive in cooperation with the quick response. The third charateristic is to obtain easily its gray scale. There are problems of temperature dependence of threshold voltage, electric voltage dependence of response speed and the like because applied voltage is controlled for obtaining the gray scale in the TN method. In comparison with the TN method, the display method applying the light switching effect of the S.sub.C * phase is suitable for graphic display because it is able to obtain the gray scale by controlling inversion time of polarity.
For such materials of ferroelectric liquid crystals having excellent characteristics, liquid crystal compounds having the S.sub.C * phase and the high value of spontaneous polarization (abbreviated as Ps hereinafter) were desired. Afterward, it was found that a ferroelectric liquid crystal mixture was obtained by adding a chiral and optically active compound, which does not show a liquid crystal phase by itself, to a smectic liquid crystal. Thus, search fields of the materials of ferroelectric liquid crystals were more widened (ref. Mol. Cryst. Liq. Cryst. 89, 327 (1982)).